Method and structure for variable pitch microwave probe assembly

ABSTRACT

A coplanar waveguide (CPW) probe includes at least one center probe element, each having a respective center probe contact point and at least one peripheral probe element, each having a respective peripheral contact point. The pitch between the at least one center contact point and the at least one peripheral contact point is adjustable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to test probes forcharacterizing microwave frequency circuits and packages. Morespecifically, a variable pitch air coplanar waveguide (CPW) structureallows the pitch of the CPW tips to be adjusted to accommodate a rangeof pad pitches with minimal degradation in the microwave performance ofthe probe launch.

2. Description of the Related Art

High performance (e.g. 40 GHz bandwidth) microwave probes are currentlyavailable for on-wafer testing of microwave frequency circuits. They arealso being used increasingly for package level characterization as well.

These probes are usually constructed of very small diametermicro-coaxial cable which is terminated on the device-under-test (DUT)end in an air coplanar waveguide (CPW) structure in aGround-Signal-Ground (G-S-G) or similar configuration. A seriousdisadvantage of these probes is their high cost (in some cases,averaging $600 per probe). Since the CPW tips are fixed in pitch, adifferent probe needs to be purchased for each significant change in padpitch or package geometry.

SUMMARY OF THE INVENTION

A representative view 100 of fixed pitch probes is shown in FIG. 1. Theleft probe 101 shows the CPW region for a Ground-Signal-Ground (G-S-G)configuration, the other two probes 102, 103 show variants in which onlya single ground contact is provided (e.g., signal-ground or S-G probes).The pitch (e.g., separation between contact points) is depicted in FIG.1 as exemplarily being 0.15 mm or 0.3 mm.

The probe tips 104,105,106 can be considered as extensions of a highfrequency micro-coaxial cable 107 in which the internal conductor shownconnected to tip 108 carries a signal and the outer jacket 109 conveysthe return current. The ground tips 110 are typically brazed or solderedto the outer wall 109 of the micro-coaxial cable 107. A variation notshown in FIG. 1 is a G-S-S-G probe, in which there are two internalsignal conductors rather than only one.

The CPW probes shown in FIG. 1 are well known in the art, and there isno need to further discuss the design details of these probes in orderto discuss the present invention.

However, it is noted that the electrical performance of any component,such as a testing probe, used at the high frequencies inherent in themicrowave range is extremely sensitive to changes in shape anddimensions. This is one reason that conventional CPW probes are fixed inpitch since it simplifies the optimization of shape and dimensions.

A key function of the CPW probe is to provide a smooth transitionbetween transmission modes. For example, in the micro-coaxial cable, thecylindrical configuration of the micro-coaxial structure sustains thetransverse electric magnetic (TEM) mode of transmission. However, theDUT may be conducting a different mode. A key function of theconventional CPW probe is to provide a smooth transition between themode in the DUT and the TEM mode in the micro-coax. Therefore, it iscritical that a CPW probe design not permit reflections or attenuationsat microwave frequencies that would disrupt this smooth transitionbetween modes.

Because of the fixed configuration of conventional microwave probes, thehigh cost for each probe, and the technical constraints of maintaining aproper transition at the probe, there are currently no CPW microwaveprobes that have an adjustable pitch for the contacts.

Although circuits fabricated on semiconductor wafers are typicallydesigned so that the pitch of the terminals meet a standard pitch, suchas the 150 μm or 300 μm pitch shown in FIG. 1, in packaging or printedcircuit cards, there are many standards and designs for the pitch.

Therefore, a need exists for a CPW microwave probe that can be adjustedfor various pitches without compromising the electrical performancerequired to be maintained at these high frequencies.

Apart from saving the inventory costs of stocking many different probetip pitches, a probe tip having adjustable pitch would enable quickmeasurements of many different package and circuit geometries to be madewithout paying the penalty of fabrication lead time for these probes.Moreover, it is often the case that the long lead time for these probescan represent a more significant economic penalty than the costs of theprobes themselves.

In view of the foregoing, and other, exemplary problems, drawbacks, anddisadvantages of the conventional system and probe, it is an exemplaryfeature of the present invention to provide a microwave probe that hasan adjustable pitch.

It is another exemplary feature of the present invention to provide amicrowave probe that has adjustable pitch and that maintains thecritical electrical interface requirements in the microwave range.

It is another exemplary feature of the present invention to reduce thecost of testing microwave components by providing a test probe that canbe adjusted for different test contact pitches, thereby precluding theneed to maintain a large inventory of probes having different pitches.

To achieve the above exemplary features and others, in a first exemplaryaspect of the present invention, described herein is a coplanarwaveguide (CPW) probe including at least one center probe element, eachhaving a respective center probe contact point, and at least oneperipheral probe element, each having a respective peripheral contactpoint, wherein a pitch of the at least one center contact and the atleast one peripheral contact is adjustable.

In a second exemplary aspect of the present invention, also describedherein is a test probe assembly including a micro-coaxial cable havingat least one center conductor and a conductive outer wall and a probetip section comprising at least one center contact, each respectivelyextending from the at least one center contact, and at least oneperipheral contact, each electrically connected to the conductive outerwall, wherein a pitch between the at least one center contact and the atleast one peripheral contact is adjustable.

In a third exemplary aspect of the present invention, also describedherein is a method of testing an electronic circuit operating in amicrowave range of wavelength, including making an adjustment of acontact pitch on an air coplanar waveguide (CPW) probe having anadjustable contact pitch and placing the contacts of the CPW probe incontact with test points of the electronic circuit.

In a fourth exemplary aspect of the present invention, also describedherein is a method of fabricating a micro-coaxial probe assembly,including attaching at least one peripheral probe tip element to anouter wall of a micro-coax assembly having at least one centerconductor, the center conductor being extended to serve as an innerprobe element for testing a test point, and at least one peripheralprobe tip element being attached to the outer wall a predetermineddistance from an end of the inner probe element to be used to contact atest point; and incorporating an interface on the outer wall for amechanism that allows the at least one peripheral probe tip element tobe adjusted in separation from the at least one center conductor.

Thus, the present invention provides a structure and method for improvedtesting of circuits operating in the microwave region by providing atest probe having an adjustable pitch that maintains the electricalaspects of the interface transition over the adjustable range, therebyproviding a universal probe that can be used for different test pointpitches.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other exemplary features, aspects and advantages willbe better understood from the following detailed description of anexemplary embodiment of the invention with reference to the drawings, inwhich:

FIG. 1 shows a view 100 of the DUT tip region of various configurationsof conventional microwave probes;

FIG. 2 shows an exemplary embodiment 200 of an adjustable microwaveprobe configuration in accordance with the concepts of the presentinvention;

FIG. 3 shows a side view 300 of the exemplary embodiment 200;

FIG. 4 shows an end view 400 of the exemplary embodiment 200;

FIG. 5 shows an exemplary fabrication process 500 of exemplaryembodiment 200; and

FIG. 6 shows an exemplary testing process 600 using the exemplaryembodiment 200.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 2-5, anexemplary embodiment 200 of the present invention will now be described.

The present invention modifies the geometry of the conventional tips ofthese microwave probes (e.g., as shown in FIG. 1) to permit the user toset the pitch between the tip contact points over a wider range whilemaintaining minimal degradation in the high frequency performance. FIG.2 shows key concepts of the present invention in greater detail, andFIGS. 3 and 4 show respective plane views looking from the perspectivesindicated in FIG. 2.

The exemplary adjustable air coplanar waveguide (CPW) probe 200 of thepresent invention includes:

a section of micro-coaxial 201, having outer wall 202 and centerconductor 203;

at least one peripheral probe element 204 (exemplarily, a probe elementfor ground) attached to the outer wall 202 such that the shape presentsa taper that leaves a variable gap 205 between the outer wall 202 andthe ground probe element 204;

a flexure member (e.g., a spreader member such as a spring) 206 thatpushes the ground probe element 204 away from the outer wall 202; and

a sleeve 207 that can move longitudinally 208 along the outer wall 202,as exemplarily movable along threads 209 on the outer wall 202, thelongitudinal movement 208 thereby causing the ground probe element(s)204 to swing in an arc 210, thereby changing pitch relative to signalprobe element 203.

In contrast to the conventional probe, the inventive probe avoids thehard attachment of the ground tips 204 at the end of the micro-coaxialnext to the probe contact area. Rather, the ground tips 204 are attachedfurther back and are formed in such a way as to permit flexure so thatthey can be separated away from the wall of the micro-coaxial invariable amounts, as shown in FIG. 2.

Additionally, the present invention includes, exemplarily, a knurledsleeve 207 which mates onto a thread 209 that is either machined intothe wall of the micro-coaxial or attached to it. Turning this sleeveresults in translation of the sleeve in a longitudinal direction. Thisaction coupled with the taper formed into the ground tip pieces eitherincreases or decreases the separation between the ground tips, therebyeffecting the change in pitch.

The present invention includes a flexure member (e.g., spring) 206 toprovide a reliable mechanism to keep the ground tip pieces 204 engagedagainst the inner surface of the knurled sleeve 207. This mechanicalfunction could be accomplished using the spring forces in the groundtips 204 alone, but the presence of the spring 206 provides a morereliable restoring force. In addition, the spring member 206 plays animportant electrical function of providing a short path for the returnground current directly from the contact points of the ground tips tothe sidewall 202 of the micro-coaxial 201. Without this short path, highfrequency performance may be compromised by a large ground inductance.

This spring 206 can be fabricated from, for example, BeCu or any othergood conductor with similar mechanical properties. It is also possibleto use a conductor-loaded elastomeric button or a “fuzz button” in thisrole as spring 206.

These features described above can also be extended so as to beincorporate in the single sided Ground-Signal or Signal-Ground probes102, 103 shown in FIG. 1. It should be apparent to one having ordinaryskill in the art, after taking the discussion herein as a whole, thatthe concepts herein could also be extended to ganged multiple tipgeometries in which there is more than one signal conductor (e.g.,G-S-S-G probes).

Additionally, it should be apparent that, depending upon the specificmanner in which the sleeve 207 is adapted to slide along the outer wall202, the outer wall 202 of the micro-coaxial may require a bit ofstiffening in the region where the threaded section 209 and the sleeve207 are located, in which case the outer wall might, for example,receive additional layer(s) of material during fabrication or have anadditional layer or layers of material added to this area especially forthe purpose of strengthening the outer wall for purpose of an interfacewith the sleeve 207.

Another possible modification to the basic concepts described abovemight include, for example, a calibration scale 211 associated withpositioning of the sleeve, so that a user can make a setting, or atleast an approximate setting, without having to view the results of theadjustment under a microscope while making the initial adjustment.

It should be apparent to one of ordinary skill in the art, after takingthis disclosure as a whole, that the calibration scale could be based oneither a calibration indication based on the longitudinal position ofthe sleeve, whereby the calibration markings 211 might be locatedsomewhere on the sidewall 202 or threaded interface 209 of themicro-coaxial cable. Alternatively, depending upon the details of theshape of the taper of the peripheral probe element 204 and the size ofthe thread 209, the calibration might more accurately be calibratedthroughout the intended range of the probe within one or two rotationsof the sleeve. In this configuration, the calibration could then beindicated by a calibration marker 212 on the sleeve 207, in combinationwith calibration markers 213 on the sidewall 202 of the micro-coaxialcable 201.

There are design tradeoffs to be considered for a probe assembly of thepresent invention. Thus, in a non-limiting example, the exemplaryconfiguration 200 might have a span of 2:1 or 3:1, meaning that onevariable pitch probe might span, for example, pitches of 50-150 microns(e.g., total GND-GND separation of 100-300 microns in the configurationshown in FIG. 2). A second exemplary probe might cover a second range of300-900 microns.

These two probes would provide coverage of most current high frequencyapplications, but it should be apparent that a wider range could beused. The design tradeoff concern is that a wider range provides a probedesign that provides “only” a few GHz with acceptable probe performance.

FIG. 5 shows an exemplary fabrication process 500, as based on modifyinga fabrication process of a conventional probe assembly such as shown inFIG. 1. Thus, for sake of illustration, it is assumed that a partialfabrication is complete for an assembly similar to micro-coaxialassembly 101, having an outer wall 109 and center conductor 108 but noperipheral probe tips 110 yet installed. In accordance with theexemplary structure shown in FIG. 2, in step 501 of FIG. 5, the twoperipheral probe tip elements 204 would be affixed to the outer wall by,for example, soldering or brazing, at a location away from the tip ofthe assembly, near the location that the sleeve 207 is to be installed.

In step 502, the interface for the sleeve is incorporated on the outerwall 202. In the exemplary configuration of FIG. 2, this interface is athreaded arrangement on the outer wall 202 that matches an internalthread on the sleeve 207, but other mechanisms are possible, as long athe sleeve 207 can move longitudinally along the taper of the peripheralprobe tip element 204.

The thread interface 209 might be achieved by machining the outer wall202 of the micro coax, or it might be achieved by mounting, attaching,or otherwise affixing, a collar that has an external threaded surface tothe outer wall 202. It might be necessary to strengthen the area of thethreaded interface, especially if the thread is machined directly intothe outer wall 202.

In step 503, the sleeve 207 is installed, by slipping it over theperipheral probe tip elements 204 and onto the sleeve interface (e.g.,screwing it onto the external screws of the thread interface). Finally,in step 204, the flexible member 206 is inserted to force the peripheralprobe tip elements 204 outward and provide the short current returnpath.

FIG. 6 shows an exemplary test process 600 using the adjustable probe ofthe present invention. In step 601, a coarse adjustment of the tip pitchis made using the alignment marks on the probe assembly (e.g., labels211, 213 in FIG. 2). In step 602, a fine adjustment is made, typicallyusing a magnification device to either compare the actual tip pitch withan accurate scale or compare the actual tip pitch against the testpoints themselves. Finally, in step 603, the test probe points areapplied to the test points, typically using a mechanical holding fixturethat holds the test probe assembly against the test points on the deviceunder test.

Therefore, because of the delicate nature of working with microwavecomponent wavelengths that typically involves using magnification forpositioning a test probe, the probe assembly of the present inventionmight well be an accessory, a detachable component, or even a built-incomponent, of a larger test fixture used for testing microwavecomponents, devices, chips, or wafers.

Apart from saving the inventory costs of stocking many different probetip pitches, the adjustable probe of the present invention also providesthe benefit of quick measurements of many different package and circuitgeometries, without paying the penalty of fabrication lead time forthese probes. It is often the case that the long lead time for theseprobes can represent a more significant economic penalty than the costof the probes themselves.

While the invention has been described in terms of exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Further, it is noted that Applicants' intent is to encompass equivalentsof all claim elements, even if amended later during prosecution.

1. A coplanar waveguide (CPW) probe assembly, comprising: at least onecenter probe element, each having a respective center probe contactpoint; and at least one peripheral probe element, each having arespective peripheral contact point, wherein a pitch of said at leastone center contact and said at least one peripheral contact isadjustable.
 2. The CPW probe assembly of claim 1, wherein a physicalseparation between said at least one center contact and said at leastone peripheral contact is controlled by a longitudinal translation of amovable sleeve fitted to an outer wall of said CPW probe assembly. 3.The CPW probe assembly of claim 1, further comprising: a spreader forurging said at least one peripheral probe element apart from said atleast one center probe element.
 4. The CPW probe assembly of claim 1,further comprising: a shorting device that maintains an electricalcontact between said at least one peripheral contact and an outer wallof said CPW probe as said pitch changes.
 5. The CPW probe assembly ofclaim 2, wherein said movable sleeve is fitted to said outer wall as athreaded mechanism.
 6. The CPW probe assembly of claim 2, furthercomprising: a calibration indication associated with a position of saidmovable sleeve.
 7. The CPW probe assembly of claim 4, wherein saidshorting device comprises a conductive material in a compressed statethat urges said at least one peripheral probe element apart from said atleast one center probe element.
 8. The CPW probe assembly of claim 7,wherein said shorting device comprises a metal spring.
 9. A test probeassembly comprising: a micro-coaxial cable having at least one centerconductor and a conductive outer wall; and a probe tip sectioncomprising at least one center contact, each respectively extending fromone of said at least one center conductor, and at least one peripheralcontact, each electrically connected to said conductive outer wall,wherein a pitch between said at least one center contact and said atleast one peripheral contact is adjustable.
 10. The test probe assemblyof claim 9, wherein said probe is operable over a microwave range ofwavelengths.
 11. The test probe assembly of claim 9, further comprising:a sleeve that is longitudinally movable along said outer wall, whereineach said at least one peripheral contact is attached to said outer wallsuch that a longitudinal movement of said sleeve causes said adjustablepitch.
 12. The test probe assembly of claim 11, wherein said outer wallincorporates a threaded interface and said sleeve includes an innerthread that engages therewith.
 13. The test probe assembly of claim 9,further comprising: a shorting device that maintains an electricalcontact between said at least one peripheral contact and said outer wallsubstantially adjacent to where said at least one peripheral contactcontacts a device under test.
 14. The test probe assembly of claim 13,wherein said shorting device comprises a spreader that urges said atleast one peripheral contact away from said at least one center contact.15. The test probe assembly of claim 14, wherein said spreader comprisesa metal spring.
 16. The test probe assembly of claim 11, wherein said atleast one peripheral contact is shaped to provide a taper so that saidlongitudinal movement of said sleeve compresses said at least oneperipheral contact by moving along said taper shape.
 17. The test probeassembly of claim 11, further comprising: a calibration scale on saidouter wall to provide an indication of a value of said pitch based on aposition of said sleeve.
 18. A method of testing an electronic circuit,said method comprising: making an adjustment of a contact pitch on anair coplanar waveguide (CPW) probe having an adjustable contact pitch;and placing contacts of said CPW probe in contact with test points ofsaid electronic circuit.
 19. The method of claim 18, wherein saidelectronic circuit operates in a microwave range of wavelength.
 20. Themethod of claim 18, wherein said making an adjustment of said contactpitch includes moving a sleeve along an outer wall of said CPW probe.21-25. (canceled)